Semiconductor device with multiple threshold voltage and method of fabricating the same

ABSTRACT

According to an exemplary embodiment, a chip is provided. The chip includes a first vertical device having a first threshold and second vertical device having a second threshold. The first vertical device includes a first source; a first channel over the first source; a first drain over the first channel; a first conductive layer adjacent to the first channel; and a first gate adjacent to the first conductive layer. The second vertical device includes a second source; a second channel over the second source; a second drain over the second channel; a second conductive layer adjacent to the second channel; and a second gate adjacent to the second conductive layer.

FIELD

This disclosure relates generally to semiconductors, and, moreparticularly, to a semiconductor device with multiple threshold voltageand a method of fabricating the same.

BACKGROUND

In the advanced technology, integration of various functions into asingle chip may shrink the dimension of the chip and enhance theperformance. Since various functions require multiple thresholds, thereis a need for a semiconductor device with multiple threshold voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating an exemplary semiconductordevice according to an exemplary embodiment.

FIG. 2 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 3 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 4 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 5 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 6 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 7 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 8 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 9 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 10 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment.

FIG. 11 is a sectional view illustrating an exemplary semiconductordevice according to an exemplary embodiment.

FIG. 12 is a block diagram illustrating an exemplary chip according toan exemplary embodiment.

FIGS. 13 a-13 h are block diagrams illustrating an exemplary chipaccording to an exemplary embodiment.

FIGS. 14 a-14 h are block diagrams illustrating an exemplary chipaccording to an exemplary embodiment.

FIG. 15 is a flow chart for manufacturing a chip comprising a firstvertical device having a first threshold and a second vertical devicehaving a second threshold according to an exemplary embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, which areillustrated in the accompanying drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

The disclosure describes a method which may utilize a forming processand an implanting process to fabricate vertical devices having multiplethresholds and is suitable for different functions. The forming processmay refer to deposition processes, photolithography processes, etchingprocesses or the combination thereof.

FIG. 1 is a sectional view illustrating an exemplary semiconductordevice according to an exemplary embodiment. As shown in FIG. 1, a chip100 is provided. In the chip 100, a first vertical device 110 having afirst threshold and a second vertical device 120 having a secondthreshold are provided over a substrate 101. The first vertical device110 and the second vertical device 120 may be vertical-gate-all-arounddevices electrically isolated by a shallow trench isolation 102. Thefirst vertical device 110 may be a PMOS, and may include an n-well 111,a first source 112, a first channel 113 and a first drain 114. Thesecond vertical device 120 may be a NMOS, and may include a p-well 121,a second source 122, a second channel 123 and a second drain 124.Salicides 115, 116, 125, 126 are used to reduce contact resistance. Thethreshold may refer to the value of the gate-source voltage when theconducting channel just begins to connect the source and drain of thetransistor, allowing significant current.

The first source 112 may be disposed over the n-well 111. The firstchannel 113 may be disposed over the first source 112. The first drain114 may be disposed over the first channel 113. The second source 122may be disposed over the p-well 121. The second channel 123 may bedisposed over the second source 122. The second drain 124 may bedisposed over the second channel 123. A silicon nitride layer 103 as aninsulator may be disposed over the first source 112, the second source122 and the shallow trench isolation 102. An oxide layer 104 may bedisposed over the silicon nitride layer 103.

FIG. 2 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 2, a high-k dielectric layer202 may be formed over a portion of the silicon nitride layer 103 andthe oxide layer 104. In another exemplary embodiment, an interfaciallayer (not shown) may be formed over a portion of the silicon nitridelayer 103 and the oxide layer 104. Then, the high-k dielectric layer 202may be formed over the interfacial layer (not shown).

FIG. 3 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 3, a first conductive layer302 may be formed over the high-k dielectric layer 202 as a portion of afirst gate electrode. The first conductive layer 302 may be formed of,for example, a p-type work function metal. The p-type work functionmetal may be formed of, for example, compounds of TiN, TaN basedmaterial with Te, Re Rh, Be, Co. The thickness of the p-type workfunction metal may be 5-100 angstrom.

In the exemplary embodiment, the first conductive layer 302 may befurther implanted with a first dopant 304 at a tilted angle 306 withreference to vertical direction. In detail, the tilted angle 306 withreference to vertical direction for implantation may be about 5 to 60degrees. The first dopant 304 may be formed of, for example, Al, Ga, In,Be, Se, F. Typical implantation energies may be, for example, in therange of 100 eV to 25 keV. Efficient dosage may be, for example, in therange of 10¹³ ions/cm² to 10¹⁶ ions/cm².

FIG. 4 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 4, the portion of the firstconductive layer 302 corresponding to the second vertical device 120 isremoved, and the portion of the first conductive layer 302 correspondingto the first vertical device 110 remains.

FIG. 5 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 5, a second conductive layer502 may be formed over the first conductive layer 302 and a portion ofthe high-k dielectric layer 202 as a portion of a second gate electrode.The second conductive layer 502 may be formed of, for example, an n-typework function metal. The n-type work function metal may be formed of,for example, compounds of TiN, TiAl, TaN based material with Al, Mn, Zr,Pb, Sn. The thickness of the n-type work function metal may be 5-300angstrom. In another exemplary embodiment, a third conductive layer maybe formed over the second conductive layer 502. The lateral surface ofthe channel 113 of the first vertical device 110.

In the exemplary embodiment, the second conductive layer 502 may befurther implanted with a second dopant 504 at a tilted angle 506 withreference to vertical direction. In detail, the tilted angle 506 withreference to vertical direction for implantation may be about 5 to 60degrees. The second dopant 504 may be formed of, for example, As, Sb, P,Ta, Te, Sn, S, Mo, Cs, Ge. Typical implantation energies may be, forexample, in the range of 100 eV to 25 keV. Efficient dosage may be, forexample, in the range of 10¹³ ions/cm² to 10¹⁶ ions/cm².

FIG. 6 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 6, a gate layer 602 may beformed over the second conductive layer 502 by deposition. The gatelayer 602 may be formed of, for example, Al, W, Co or Cu.

FIG. 7 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 7, a chemical mechanicalpolishing is performed on the gate layer 602 and stops on the firstconductive layer 302.

FIG. 8 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 8, a dry etch back process isperformed on the gate layer 602, high-k dielectric layer 202, the firstconductive layer 302 and the second conductive layer 502 to expose thefirst drain 114 and the second drain 124.

FIG. 9 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 9, a dry etch back process isperformed on the gate layer (not shown), high-k dielectric layer 202,the first conductive layer 302 and the second conductive layer 502 toexpose a portion of the oxide layer 104 and to form a gap 902. A firstgate metal 904 is formed adjacent to the first conductive layer 302 andthe second conductive layer 502, and a second gate metal 906 is formedadjacent to the second conductive layer 502.

FIG. 10 is a sectional view illustrating the exemplary chip according tothe exemplary embodiment. As shown in FIG. 10, the oxide layer 104 maybe re-formed over the first vertical device 110 and the second verticaldevice 120. And a chemical mechanical polishing is performed on theoxide layer 104 and stops on the salicides 115, 125.

As such, in the chip 100, the first vertical device 110 having the firstthreshold and the second vertical device 120 having the second thresholdare formed. The first conductive layer 302 and the second conductivelayer 502 of the first vertical device 110 corresponds to the firstthreshold, and the second conductive layer 502 of the second verticaldevice 120 corresponds to the second threshold.

FIG. 11 is a sectional view illustrating an exemplary semiconductordevice according to an exemplary embodiment. As shown in FIG. 11, a chip1100 is provided. In the chip 1100, a first vertical device 1110 havinga first threshold and a second vertical device 1120 having a secondthreshold are provided over a substrate 1101. The first vertical device1110 and the second vertical device 1120 may be electrically isolated bya shallow trench isolation 1102. The first vertical device 1110 may be aPMOS, and may include a n-well 1111, a first source 1112, a firstchannel 1113, a first drain 1114, a first gate metal 1131, a firstconductive layer 1132 and a first high-k dielectric layer 1133. Thesecond vertical device 1120 may be a NMOS and may include a p-well 1121,a second source 1122, a second channel 1123, a second drain 1124, asecond gate metal 1141, a second conductive layer 1142 and a secondhigh-k dielectric layer 1143. Salicides 1115, 1116, 1125, 1126 are usedto reduce contact resistance. The threshold may refer to the value ofthe gate-source voltage when the conducting channel just begins toconnect the source and drain of the transistor, allowing significantcurrent.

The first source 1112 may be disposed over the n-well 1111. The firstchannel 1113 may be disposed over the first source 1112. The first drain1114 may be disposed over the first channel 1113. The second source 1122may be disposed over the p-well 1121. The second channel 1123 may bedisposed over the second source 1122. The second drain 1124 may bedisposed over the second channel 1123. A silicon nitride layer 1103 asan insulator may be disposed over the first source 1112, the secondsource 1122 and the shallow trench isolation 1102. An oxide layer 1104may be disposed over the silicon nitride layer 1103.

The first conductive layer 1132 is disposed adjacent to the firstchannel 1113, and the second conductive layer 1142 is disposed adjacentto the second channel 1123. The first high-k dielectric layer 1133 maybe disposed between the first conductive layer 1132 and the firstchannel 1113, and the second high-k dielectric layer 1143 may bedisposed between the second conductive layer 1142 and the second channel1123. The first gate metal 1131 is disposed adjacent to the firstconductive layer 1132, and the second gate metal 1141 is disposedadjacent to the second conductive layer 1142.

In an exemplary embodiment, the first conductive layer 1132 and thesecond conductive layer 1142 may be formed of a first material, thefirst conductive layer 1132 is implanted with a first dopant, and thesecond conductive layer 1142 is implanted with a second dopant. Thefirst dopant may be formed of, for example, Al, Ga, In, Be, Se, F. Thesecond dopant may be formed of, for example, As, Sb, P, Ta, Te, Sn, S,Mo, Cs, Ge. Typical implantation energies may be, for example, in therange of 100 eV to 25 keV. Efficient dosage may be, for example, in therange of 10¹³ ions/cm² to 10¹⁶ ions/cm².

In an exemplary embodiment, the first conductive layer 1132 and thesecond conductive layer 1142 may be formed of a first material, thefirst conductive layer 1132 is implanted with a first dopant with afirst concentration, and the second conductive layer 1142 is implantedwith the first dopant with a second concentration. In an exemplaryembodiment, the first conductive layer 1132 and the second conductivelayer 1142 may be formed of a first material, the first conductive layerhas a first thickness, and the second conductive layer has a secondthickness. In an exemplary embodiment, the first conductive layer 1132is formed of a first material having a first work function, and thesecond conductive layer 1142 is formed of a second material having asecond work function. In the above exemplary embodiments, the firstconductive layer 1132 corresponds to the first threshold, and the secondconductive layer 1142 corresponds to the second threshold.

In another exemplary embodiment, the first vertical device may furtherinclude a third conductive layer (not shown) between the first gatemetal 1131 and the first conductive layer 1132. The first conductivelayer and the third conductive layer correspond to the first threshold,and the second conductive layer corresponds to the second threshold.

FIG. 12 is a block diagram illustrating an exemplary chip according toan exemplary embodiment. As shown in FIG. 12, a chip 1200 is provided.The chip 1200 includes a first SRAM block 1202, a second SRAM block1204, an I/O logic block 1206, a function logic block 1208, an embeddedDRAM block 1210 and a flash block 1212. Said first vertical device andsaid second vertical device in the disclosure may be used for at leastone of the blocks 1202-1212.

FIGS. 13 a-13 h are block diagrams illustrating an exemplary chipaccording to an exemplary embodiment. A chip 1300 is provided.Transistors 1301-1306 are vertical devices to be manufactured to havesix different thresholds. For example, the transistors 1301-1303 arerespectively NFETs having a low threshold, a medium threshold and a highthreshold. For example, 1304-1306 are respectively PFETs having a lowthreshold, a medium threshold and a high threshold.

In FIG. 13 a, for the transistors 1301-1306, a first p-type metal layer1311 is formed over a channel layer (not shown). In FIG. 13 b, for thetransistor 1301, a first n-type metal layer 1312 is formed. In FIG. 13c, for the transistors 1301-1302, a second n-type metal layer 1313 isformed. In FIG. 13 d, for the transistors 1301-1303, a third n-typemetal layer 1314 is formed. In FIG. 13 e, for the transistors 1301-1304,a second p-type metal layer 1315 is formed. In FIG. 13 f, for thetransistors 1301-1305, a third p-type metal layer 1316 is formed. InFIG. 13 g, for the transistors 1301-1306, a cap metal layer 1316 isformed. In FIG. 13 h, for the transistors 1301-1306, a gate metal layer1318 is formed. As such, the transistors 1301-1306 having six differentthresholds are provided.

FIGS. 14 a-14 h are block diagrams illustrating an exemplary chipaccording to an exemplary embodiment. A chip 1400 is provided.Transistors 1401-1406 are vertical devices to be manufactured to havesix different thresholds. For example, the transistors 1401-1403 arerespectively NFETs having a low threshold, a medium threshold and a highthreshold. For example, 1404-1406 are respectively PFETs having a lowthreshold, a medium threshold and a high threshold.

In FIG. 14 a, for the transistors 1401-1406, a first p-type metal layer1411 is formed over a channel layer (not shown). In FIG. 14 b, for thetransistor 1401-1403, a first n-type metal layer 1412 is formed. In FIG.14 c, for the transistor 1401, a first dopant 1413 is implanted in thefirst n-type metal layer 1412. In FIG. 14 d, for the transistors1401-1402, a second dopant 1414 is implanted in the first n-type metallayer 1412. In FIG. 14 e, for the transistors 1401-1406, a cap metallayer 1415 is formed. In FIG. 14 f, for the transistor 1404, a thirddopant 1416 is implanted in the cap metal layer 1415. In FIG. 14 g, forthe transistors 1404-1405, a fourth dopant 1417 is implanted in the capmetal layer 1415. In FIG. 14 h, for the transistors 1401-1406, a gatemetal layer 1418 is formed over the cap metal layer 1415. As such, thetransistors 1401-1406 having six different thresholds are provided.

FIG. 15 is a flow chart for manufacturing a chip comprising a firstvertical device having a first threshold and a second vertical devicehaving a second threshold according to an exemplary embodiment. As shownin FIG. 15, a method 1500 is provided. The method 1500 includes thefollowing operations: forming a first conductive layer over a lateralsurface of a channel of the first vertical device (1502); and forming asecond conductive layer over a lateral surface of a channel of thesecond vertical device (1504).

In the exemplary embodiment, the method 1500 may include: implanting afirst dopant at a tilted angle with reference to vertical direction. Inthe exemplary embodiment, the method 1500 may include: forming a firstgate adjacent to the first conductive layer, and forming a second gateadjacent to the second conductive layer. In the exemplary embodiment,the method 1500 may include: forming a third conductive layer over thelateral surface of the channel of the first vertical device.

In the exemplary embodiment, the operation 1502 of forming the firstconductive layer over the lateral surface of the channel of the firstvertical device further comprises: forming the first conductive layercorresponding to the first threshold, and the operation 1504 of formingthe second conductive layer over the lateral surface of the channel ofthe second vertical device further comprises: forming the secondconductive layer corresponding to the second threshold. In the exemplaryembodiment, the operation of implanting the first dopant at the tiltedangle with the reference of the vertical direction further comprises:implanting the first dopant with 5 to 60 degrees with reference tovertical direction. In the exemplary embodiment, the operation offorming the first conductive layer and the third conductive layer overthe lateral surface of the channel of the first vertical device furthercomprises: forming the first conductive layer and the third conductivelayer corresponding to the first threshold, and forming the secondconductive layer over the lateral surface of the channel of the secondvertical device further comprises: forming the second conductive layercorresponding to the second threshold.

According to an exemplary embodiment, a chip is provided. The chipincludes a first vertical device having a first threshold and secondvertical device having a second threshold. The first vertical includes:a first source; a first channel over the first source; a first drainover the first channel; a first gate dielectric adjacent to the firstchannel; a first conductive layer adjacent to the first gate dielectric;and a first gate metal adjacent to the first conductive layer. Thesecond vertical includes: a second source; a second channel over thesecond source; a second drain over the second channel; a second gatedielectric adjacent to the second channel; a second conductive layeradjacent to the second gate dielectric; and a second gate metal adjacentto the second conductive layer.

According to an exemplary embodiment, a method for manufacturing a chipcomprising a first vertical device having a first threshold and a secondvertical device having a second threshold is provided. The methodincludes the following operations: forming a first conductive layer overa lateral surface of a channel of the first vertical device; and forminga second conductive layer over a lateral surface of a channel of thesecond vertical device.

According to an exemplary embodiment, a chip is provided. The chipincludes a first vertical device having a first threshold, secondvertical device having a second threshold and a third vertical devicehaving a third threshold. The first vertical includes: a first source; afirst channel over the first source; a first drain over the firstchannel; a first gate dielectric adjacent to the first channel; a firstconductive layer adjacent to the first gate dielectric; and a first gatemetal adjacent to the first conductive layer. The second verticalincludes: a second source; a second channel over the second source; asecond drain over the second channel; a second gate dielectric adjacentto the second channel; a second conductive layer adjacent to the secondgate dielectric; and a second gate metal adjacent to the secondconductive layer. The third vertical device having a third thresholdincludes: a third source; a third channel over the third source; a thirddrain over the third channel; a third gate dielectric adjacent to thethird channel; a third conductive layer adjacent to the third gatedielectric; and a third gate metal adjacent to the third conductivelayer.

In the exemplary embodiment, the first conductive layer corresponds tothe first threshold, the second conductive layer corresponds to thesecond threshold, and the third conductive layer corresponds to thethird threshold.

This written description uses examples in the disclosure to: disclosethe best mode and also to enable a person ordinarily skilled in the artto make and use the disclosure. The patentable scope may include otherexamples that occur to those skilled in the art.

One skilled in the relevant art upon reading this disclosure willrecognize that the various embodiments may be practiced without one ormore of the specific details, or with other replacement and/oradditional methods, materials, or components. Well-known structures,materials, or operations may not be shown or described in detail toavoid obscuring aspects of various embodiments. Various embodimentsshown in the figures are illustrative example representations and arenot necessarily drawn to scale. Particular features, structures,materials, or characteristics may be combined in any suitable manner inone or more embodiments. Various additional layers and/or structures maybe included and/or described features may be omitted in otherembodiments. Various operations may be described as multiple discreteoperations in turn, in a manner that is most helpful in understandingthe disclosure. However, the order of description should not beconstrued as to imply that these operations are necessarily orderdependent. In particular, these operations need not be performed in theorder of presentation. Operations described herein may be performed in adifferent order, in series or in parallel, than the describedembodiment. Various additional operations may be performed and/ordescribed. Operations may be omitted in additional embodiments.

This written description and the following claims may include terms,such as left, right, top, bottom, over, under, upper, lower, first,second, etc. that are used for descriptive purposes only and are not tobe construed as limiting. For example, terms designating relativevertical position may refer to a situation where a device side (oractive surface) of a substrate or integrated circuit is the “top”surface of that substrate; the substrate may actually be in anyorientation so that a “top” side of a substrate may be lower than the“bottom” side in a standard terrestrial frame of reference and may stillfall within the meaning of the term “top.” The term “on” as used herein(including in the claims) may not indicate that a first layer “on” asecond layer is directly on and in immediate contact with the secondlayer unless such is specifically stated; there may be a third layer orother structure between the first layer and the second layer on thefirst layer. The embodiments of a device or article described herein maybe manufactured, used, or shipped in a number of positions andorientations. Persons skilled in the art will recognize variousequivalent combinations and substitutions for various components shownin the figures.

What is claimed is:
 1. A chip, comprising: a first vertical devicehaving a first threshold, comprising: a first source; a first channelover the first source; a first drain over the first channel; a firstgate dielectric adjacent to the first channel; a first conductive layeradjacent to the first gate dielectric; and a first gate metal adjacentto the first conductive layer; and a second vertical device having asecond threshold, comprising: a second source; a second channel over thesecond source; a second drain over the second channel; a second gatedielectric adjacent to the second channel; a second conductive layeradjacent to the second gate dielectric; and a second gate metal adjacentto the second conductive layer.
 2. The chip of claim 1, wherein thefirst vertical device is used for at least one of an SRAM block, an I/Ologic block, a function logic block, an embedded DRAM block and a flashblock.
 3. The chip of claim 1, wherein the second vertical device isused for at least one of an SRAM block, an I/O logic block, a functionlogic block, an embedded DRAM block and a flash block.
 4. The chip ofclaim 1, wherein the first vertical device and the second verticaldevice are vertical-gate-all-around devices.
 5. The chip of claim 1,wherein the first conductive layer and the second conductive layer areformed of a first material, the first conductive layer is implanted witha first dopant, and the second conductive layer is implanted with asecond dopant.
 6. The chip of claim 1, wherein the first conductivelayer and the second conductive layer are formed of a first material,the first conductive layer is implanted with a first dopant with a firstconcentration, and the second conductive layer is implanted with thefirst dopant with a second concentration.
 7. The chip of claim 1,wherein the first conductive layer and the second conductive layer areformed of a first material, the first conductive layer has a firstthickness, and the second conductive layer has a second thickness. 8.The chip of claim 1, wherein the first conductive layer is formed of afirst material having a first work function, and the second conductivelayer is formed of a second material having a second work function. 9.The chip of claim 1, wherein the first conductive layer corresponds tothe first threshold, and the second conductive layer corresponds to thesecond threshold.
 10. The chip of claim 1, wherein the first verticaldevice further comprises a third conductive layer between the first gatemetal and the first conductive layer.
 11. The chip of claim 10, whereinthe first conductive layer and the third conductive layer correspond tothe first threshold, and the second conductive layer corresponds to thesecond threshold.
 12. A chip, comprising: a first vertical device havinga first threshold, comprising: a first source; a first channel over thefirst source; a first drain over the first channel; a first gatedielectric adjacent to the first channel; a first conductive layeradjacent to the first gate dielectric; and a first gate metal adjacentto the first conductive layer; a second vertical device having a secondthreshold, comprising: a second source; a second channel over the secondsource; a second drain over the second channel; a second gate dielectricadjacent to the second channel; a second conductive layer adjacent tothe second gate dielectric; and a second gate metal adjacent to thesecond conductive layer; a third vertical device having a thirdthreshold, comprising: a third source; a third channel over the thirdsource; a third drain over the third channel; a third gate dielectricadjacent to the third channel; a third conductive layer adjacent to thethird gate dielectric; and a third gate metal adjacent to the thirdconductive layer.
 13. The chip of claim 12, wherein the first conductivelayer corresponds to the first threshold, the second conductive layercorresponds to the second threshold, and the third conductive layercorresponds to the third threshold.
 14. A method for manufacturing achip comprising a first vertical device having a first threshold and asecond vertical device having a second threshold, comprising: forming afirst conductive layer over a lateral surface of a channel of the firstvertical device; and forming a second conductive layer over a lateralsurface of a channel of the second vertical device.
 15. The method ofclaim 14, wherein forming the first conductive layer over the lateralsurface of the channel of the first vertical device further comprises:forming the first conductive layer corresponding to the first threshold,and forming the second conductive layer over the lateral surface of thechannel of the second vertical device further comprises: forming thesecond conductive layer corresponding to the second threshold.
 16. Themethod of claim 14, further comprising: implanting a first dopant at atilted angle with reference to vertical direction.
 17. The method ofclaim 16, wherein implanting the first dopant at the tilted angle withthe reference of the vertical direction further comprises: implantingthe first dopant with 5 to 60 degrees with reference to verticaldirection.
 18. The method of claim 14, further comprising: forming afirst gate adjacent to the first conductive layer, and forming a secondgate adjacent to the second conductive layer.
 19. The method of claim14, further comprising: forming a third conductive layer over thelateral surface of the channel of the first vertical device.
 20. Themethod of claim 19, wherein forming the first conductive layer and thethird conductive layer over the lateral surface of the channel of thefirst vertical device further comprises: forming the first conductivelayer and the third conductive layer corresponding to the firstthreshold, and forming the second conductive layer over the lateralsurface of the channel of the second vertical device further comprises:forming the second conductive layer corresponding to the secondthreshold.